A spark plug used in a combustion chamber of an internal combustion chamber includes, for example, a center electrode which extends in an axial direction, an insulator which is provided to surround an outer circumference of the center electrode, a cylindrical metal shell which is assembled to an outside of the insulator and a ground electrode which is joined to a front end portion of the metal shell at a proximal end portion thereof. Then, by applying a high voltage to the center electrode, an electric spark is generated in a spark gap defined between the center electrode and the ground electrode, as a result of which a compressed air-fuel mixture is ignited.
In addition, there is proposed a technique in the patent literature in which a compressed air-fuel mixture is ignited by introducing alternating current power into a spark gap.
However, the technique described in the patent literature specifies nothing about the alternating current power that is introduced into the spark gap, leading to fears that the ignitability cannot be increased.